Kernel: Add a Timer class for aarch64

For now, this can only query microseconds since boot.

Use this to print a timestamp every second. This busy-loops
until a second has passed. This might be a good first use of
interrupts soon.

qemu used to not implement this timer at some point, but
it seems to work fine even in qemu now (qemu v 5.2.0).

Kernel/Prekernel/Arch/aarch64/Timer.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Prekernel/Arch/aarch64/MMIO.h>
+#include <Kernel/Prekernel/Arch/aarch64/Timer.h>
+
+namespace Prekernel {
+
+// "12.1 System Timer Registers" / "10.2 System Timer Registers"
+struct TimerRegisters {
+    u32 control_and_status;
+    u32 counter_low;
+    u32 counter_high;
+    u32 compare[4];
+};
+
+// Bits of the `control_and_status` register.
+// See "CS register" in Broadcom doc for details.
+enum FlagBits {
+    SystemTimerMatch0 = 1 << 0,
+    SystemTimerMatch1 = 1 << 1,
+    SystemTimerMatch2 = 1 << 2,
+    SystemTimerMatch3 = 1 << 3,
+};
+
+Timer::Timer()
+    : m_registers(MMIO::the().peripheral<TimerRegisters>(0x3000))
+{
+}
+
+Timer& Timer::the()
+{
+    static Timer instance;
+    return instance;
+}
+
+u64 Timer::microseconds_since_boot()
+{
+    u32 high = m_registers->counter_high;
+    u32 low = m_registers->counter_low;
+    if (high != m_registers->counter_high) {
+        high = m_registers->counter_high;
+        low = m_registers->counter_low;
+    }
+    return (static_cast<u64>(high) << 32) | low;
+}
+
+}